Construction of a Cooper-Pair Heterostructure

Among unconventional superconductors, Sr₂RuO₄ (SRO) has attracted tremendous attention due to the potential application in quantum computing. Although discovered for 30 years, a lot of mysteries, including order parameters are still not resolved. One of the reasons is that SRO is notoriously sensitive to even the smallest amount of impurities and dislocations. For example, even a slight surface reconstruction of SRO is sufficient to suppress the superconducting state, imposing great challenges to surface sensitive studies such as STM and ARPES to reveal its order parameter. For the same reason, a two-dimensional (2D) state in SRO is not accessible so far. In this work, we present the construction of a Cooper-pair heterostructure Ba:SRO/SRO/Ba:SRO, which possess the same single electron state throughout. The minimal Ba doping almost does not affect crystal lattice or single-electronic structure of SRO above T_C, but locally breaks Cooper pairs below T_C, leaving behind a thin layer of superfluid in the middle layer of the heterostructure. We accessed a 2D superconducting state in these heterostructures and observed a resistive regime in the temperature-field phase diagram, due to the vortex liquid state, as a signature of the 2D superconductivity. In contrast to conventional 2D superconductors, the vortex liquid state in this case is very robust against magnetic field. We attribute such robustness to the very large vortex sizes due to the unconventionality of SRO.